Crossing drones with satellites

High-altitude pseudo-satellites are a novel type of aerial vehicle designed to bridge the gap between drones and satellites, and is enabled through the maturation of sensor, battery and other technologies.

The European Space Agency (ESA) is considering extending its activities to a new region of the sky via a novel type of aerial vehicle, a missing link between drones and satellites.

High-altitude pseudo-satellites (HAPS) are platforms that float or fly at high altitude like conventional aircraft but operate more like satellites – except that rather than working from space they can remain in position inside the atmosphere for weeks or even months, offering continuous coverage of the territory below.

The best operating altitude is about 20 km above the clouds and jet streams, and 10 km above commercial airliners, where wind speeds are low enough for them to hold position for long periods.

From such a height they can survey the ground to the horizon 500 km away, enabling precise monitoring and surveillance, high-bandwidth communications or back up to existing satellite navigation services.

Fig. 2: The Zephyr, which in 2010 achieved a 14-day continuous flight without refuelling.

Several ESA directorates have teamed up to investigate their potential. For earth observation, these craft could provide prolonged high-resolution coverage for priority regions, while for navigation and telecoms they could shrink blind spots in coverage and combine wide bandwidth with negligible signal delay. The agency is looking into how these various domains can be best brought together.

While the concept has been considered for the last 20 years, it is now finally becoming a reality which is brought about by the maturing of key technologies: miniaturised avionics, high-performance solar cells, lightweight batteries and harness, miniaturisation of earth observation sensors, and high-bandwidth communication links that can deliver competitively priced services.

There is obvious potential for emergency response. HAPS could also be employed semi-permanently, perhaps extending satnav coverage into high, narrow valleys and cities.

Fig. 3: A solar-panel-topped Stratobus airship can carry up to 250 kg.

European companies have already worked on product lines of HAPS. For instance, Airbus has developed the winged, solar-powered Zephyr, which in 2010 achieved a world record 14-day continuous flight without refuelling. Zephyr-S is designed to fly payloads of a few tens of kilograms for up to three months at a time, with secondary batteries employed to keep it powered and aloft overnight. A larger Zephyr-T version in preparation will support larger payloads and power needs.

Thales Alenia Space is meanwhile preparing the lighter-than-air Stratobus, with its first flight expected in 2021. The buoyant Stratobus airship can carry up to 250 kg, its electric engines flying against the breeze to hold itself in position, relying on fuel cells at night.

Many other firms are also developing vehicles, payloads and services. These firms gathered at ESA’s inaugural workshop in October 2017, together with representatives of potential customers, including the European Defence Agency, Frontex – the EU agency tasked with Europe’s border management – and EU Copernicus environmental monitoring services. This was the first meeting of its kind in Europe, with more than 200 HAPS experts.

Fig. 4: A larger Zephyr variant to support larger payloads and power needs.

The workshop provided the agency the opportunity to learn more from the firms about the needs, opportunities and critical issues within this field, particularly as a complement for existing satellite services, to start preparing a future ESA programme. ESA regards these vehicles as a valuable way of establishing applications that complement its satellites while also accelerating space technologies through early, high-altitude flight testing.

Wider market acceptance of HAPS will come down to their efficiency and cost-effectiveness – and the best way to show that is through demonstration projects. “We have to fly them,” said Alvaro Rodriquez of the EU’s Satellite Centre. “The technology is there, all the ingredients are there, now it’s time to mix them into a nice recipe.”